The present invention relates to a formation process of an oxide layer, and especially to an anisotropic formation process of an oxide layer in a trench.
The semiconductor memory, such as a Dynamic Random Access Memory (DRAM), is usually composed of multiple memory units with storage nodes. The DRAM nowadays generally adopts a trench construction to improve capacitance in shrank memory units. The storage node is accessed through an access transistor, e.g. a vertical transistor, to allow the charge being stored into or retrieved form the storage node. The storage node of memory must be electrically isolated from the gate conductive layer.
A conventional method for ensuring electrical isolation of the storage node is forming a trench top isolation layer over the storage node. The storage node usually includes a trench partially filled with polysilicon. An oxide layer, usually a silicon dioxide, is deposited on the surface of the semiconductor device and also on the aforesaid polysilicon material. The oxide layer on the semiconductor device, except that in the trench, is removed by planarizing the surface. Afterwards, the oxide layer in the trench is etched and a predetermined thickness thereof remains. However, the oxide layer alone may not provide sufficient and reliable electrical isolation. Additionally, the oxide layer etching can not be controlled easily and the thickness of the residual oxide layer varies substantially. It is noted the substantial thickness variation of the remained oxide layer is not allowed in DRAM manufacturing processes.
When a vertical transistor is fabricated in a memory device, a buried strap portion of the storage node, which is under the top trench oxide, must diffuse out and electrically connect to the vertical transistor channel. Therefore, if the transistor is xe2x80x9cONxe2x80x9d, a bitline electrically connects with the storage node. The channel must electrically isolate from the gate conductive layer through an insulating layer. The insulating layer is usually formed by partially oxidizing polysilicon of the gate oxide layer in the trench.
Many prior art works in this field, such as U.S. Pat. No. 6,184,091 issued to Gruening et al. entitled xe2x80x9cFormation of Controlled Trench Top Isolation Layers for Vertical Transistorsxe2x80x9d, are disclosed. Please refer to FIG. 1a, a semiconductor device 100 has a substrate 101, a pad oxide layer 102 and a pad nitride layer 103 thereon. A trench 104 is formed deep into the substrate 101, the pad oxide layer 102 and pad nitride layer 103. A collar 105 is formed in the trench 104 for electrically isolating a portion of trench 104 from the substrate 101. The trench 104 is partially filled with a conductive filler material 106. The filler material 106 extends over the top portion of collar 105 and contacts with the substrate 101. Therefore, a buried strap 108 is formed in the trench. A recess 107 remains at the top of the trench 104. Looking at the recess 107, the recess 107 includes a bottom surface 109 and a circumferential wall 110. The bottom surface 109 is the top surface of buried strap 108.
Please refer to FIG. 1b, the trench top oxide formation starts with a nitride liner 111 formed on the circumferential wall 110. The nitride liner 111 formation is first deposited a nitride liner 111 on the pad nitride layer 103, bottom surface 109 and circumferential wall 110. The nitride liner 111 is preferably a silicon nitride. Then, the nitride liner 111 is removed from all surfaces except the circumferential wall 110. A sub-atmospheric chemical vapor deposition (SACVD) layer 112 is then deposited. The SACVD layer is usually an oxide or an ozone rich TEOS (tetraethoxysilane) layer. The SACVD layer 112 grows at a rate of about 5 times greater on silicon than on nitride. Therefore, the thickness of the SACVD layer 112 is about 5 times thicker on the bottom surface 109 than on the pad nitride layer 103 and circumferential wall 110. Accordingly, it is easy that the SACVD layer 112 on the pad nitride layer 103 and circumferential wall 110 are removed, and on the bottom surface 109 remains the SACVD layer 112 of reduced thickness, as shown in FIG. 1c. In the succeeding processes, the nitride liner 111 is removed and an oxide layer 113 is formed to serve as a gate oxide on the circumferential wall 110, as shown in FIG. 1d. 
As mentioned above, the fabrication process of a trench top oxide is complicated. The steps of first depositing the SACVD layer 112, and then etching have multiple variables to control while etching such that stability is low. Before forming the gate oxide 113, the nitride linear 111 must be removed in advance. To improve yield, and to decrease number of steps, it has a demand for a new method for forming trench top oxide and gate oxide with less steps and higher yield.
An objective of the present invention is to provide a method for forming a trench top oxide and a gate oxide in fewer steps.
Another objective of the present invention is to provide a method for forming the trench top oxide and gate oxide at the same time.
The method of the present invention is to form a thick oxide layer on a bottom surface of a recess and a thin oxide layer on a circumferential wall of the recess. The recess is part of a trench formed in a substrate, and the circumferential wall is perpendicular to the bottom surface. The method includes steps as follows. Inert gas ions are implanted into the bottom surface of the recess at a direction parallel to the circumferential wall. Afterwards, the substrate is thermally processed by using a thermal oxidation process to form a thick oxide layer on the bottom surface and a thin oxide layer on the circumferential wall.